I made this request several years ago and nothing happened.
We have had to import orders and map the import so our Ship by Date is mapped to the Paid date. Which is confusing.
If Shipstation wont add the field to the import, which makes no sense, then at least give more options in automation instead of a fixed number of days.
For example X number of days from paid date or X number of days from Custom field 1, 2 or 3.
That way i can use 0 days from Paid date. However that still doesn't resolve the issue where i can use the paid date.
That is a painful workaround. Being in the tree nursery business we obviously can't ship trees to people who still have snow on the ground. So we need to import the ship by date that was entered by the customer and in a perfect world we'd like to see it put in hold status until that date. This seems like an easy feature to add.